Composite network strands

Flory ( 9) has treated the interesting case of subsequent removal of the first stage cross-links without chain scission. Even after complete removal, i.e. =0, there is still a certain memory of the structure of the first network since the composite network strands were physically part of both networks. According to Flory s theory, the resulting network may be treated as if a certain fraction, 0, of the strands of the second network were effectively converted into strands of the first network, >ie. 

Zhang et al. studied the effect of conductive network formation in a polymer melt on the conductivity of MWNT/TPU composite systems (91). An extremely low percolation threshold of 0.13 wt% was achieved in hot-pressed composite film samples, whereas a much higher CNT concentration (3-4 wt%) is needed to form a conductive network in extruded composite strands. This was explained in terms of the dynamic percolation behavior of the CNT network in the polymer melt. The conductivity of extruded strand showed a hopping resistivity dominated behavior at low concentrations and a dynamic percolation induced network dominated behavior at higher concentrations. It was shown that a higher temperature can reduce the filler concentration required for the dynamic percolation to take effect. 

Chopped strands several inches long can be loosely bound as a mat that is porous and in which the strands are randomly oriented in two dimensions. This form is suitable for impregnation by a liquid polymer. After polymerization or cross-hnldng (curing) under pressure, the composite will comprise a polymer-network matrix in which the individual strands are embedded. 

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